1. Field of the Invention
The present invention relates to a CD-ROM reproducing apparatus for reading out data recorded on the surface of a compact disk (CD), and particularly to a CD-ROM reproducing apparatus of the type which can be connected to an external computer system and used, and which can read out both computer data (CD-ROM data) and music data (CD-DA data) on a CD. More particularly, the present invention relates to a CD-ROM reproducing apparatus of the type which can read out computer data even during the reproduction of music data.
2. Description of the Prior Art
With the recent technological improvement, various kinds of personal computers (PC) such as desktop types and notebook types have been developed and sold on the market. These computer systems have a CPU and a memory as essential components, and also, as a standard or optional feature, a non-volatile external storage device in order to record and store a large capacity of program or data. The external storage device is, for example, a FDD (Floppy Disk Drive), a HDD (Hard Disk Drive), a CD-ROM (Compact Disk-Read Only Memory) drive, or the like. For example, in the case of an "IBM Thinkpad 560" ("Thinkpad" is a trademark of IBM Corporation) marketed by IBM Japan Corporation, not only a HDD is normally provided, but also an external FDD can be optionally connected to a FDD port located in the rear part of a PC main body, or an external CD-ROM drive can be optionally connected via a PC card (IDE card) (see FIG. 1). An example of the external CD-ROM drive is "CD-400" marketed by IBM Japan Inc.
Since the CD (compact disk) can record a large capacity with high density, it is mainly applied to recording enormously large information. Especially recently, a CD-ROM drive has been rapidly popularized, since the CD-ROM has been compact and its access time has been decreased as well as since software distribution by means of the CD-ROM has been usually effected as a result of operating system (OS) or applications being enlarged.
The CD is an information storage medium which comprises a disk made of a transparent resin as a substrate, on one surface of which pits corresponding to digital data (protruding and recessed forms with different reflectance) are spirally formed with the same density, an extremely thin metal film such as aluminum is fixed thereon, and a hard protective film is further formed on its upper layer. Then, the CD-ROM drive (CD-ROM reproducing apparatus) for reproducing data stored on the disk reads out the data by emitting laser light on the rotating disk and using the change of the intensity of reflected light. A scanning operation on the disk surface is usually carried out in accordance with a constant linear velocity (CLV) method. The scanning velocity is defined based on a "standard speed" which is equivalent to the speed of reading out and reproducing audio data. Recently, a CD-ROM drive capable of reading out the data at two times speed, four times speed, and further, eight times speed has also made an appearance.
Physical and logical specifications of a CD were proposed mainly by Sony Inc. and Philiphis Corp. and the specific contents thereof were described in a so-called "Red Book", "Yellow Book", "Green Book", and the like.
Data is basically recorded in a spiral manner on the CD from a part near the center of a disk surface to its outer periphery with the same density (see FIG. 9(a)). The physical format of the data on the CD comprises three areas of a Lead In area, a program area, and a Lead Out area in the direction from its inside to its outside. The area enclosed between Lead In and Lead Out is called a "session". A multi-session type CD has a plurality of program areas which are separated from the Lead In area and Lead Out area.
The Lead In and Lead Out areas are provided for defining a program area, and recording the attribute information of the disk or various kinds of control information such as the start position of each track in the program area. The program area is an area in which the substantial user data such as computer data or audio data is stored and comprises one or a plurality of tracks. A track can be further divided into sectors having the length of 2K byte. Each track has a variable length (Namely, the number of sectors in one track is not constant) and its length depends on the length of user data to be stored. For example, in the case of music CD, one track corresponds to one tune and the number of sectors in the track corresponds to the performance time of the tune.
Generally, while computer data is stored in the CD in accordance with a "CD-ROM format", audio data is stored in accordance with a "CD-DA format" (track of the CD-ROM format is referred to as a "CD-ROM track", and a track of the CD-DA format is referred to as a "CD-DA track", hereinafter). As one kind of the CD-DA format, there is a "CD-Graphics" format in which image data is stored in the free area of the audio data. Additionally, other formats also exist, including a "CD-I (Compact Disk-Interactive media)" format in which image data and audio data are stored in the same track in accordance with a time-division multiplexing of multiple channels) and a "CD-ROM XA" format or the like. Recently, a hybrid type CD, has been also widely employed, in which CD-ROM tracks and CD-DA tracks are mixed in a single CD.
FIG. 9(b) schematically shows the physical format of a CD. As illustrated, one session is formed in such a manner that a program area which stores the substance of user data is sandwiched between a lead in area and a lead out area.
The program area can include up to 99 tracks at maximum. In the case of the CD-DA track, one track corresponds to one tune, and in the case of music CD, the number of tracks corresponds to the number of stored tunes. The length of the track is not constant. For example, as for the CD-DA track, the number of sectors included in one track depends on the performance time of a tune.
Various kinds of control information for a session are recorded in the lead in area and the lead out area. A table for managing attribute information and control information in the session for each track unit, which is called a TOC (Table of Contents), is stored in the lead in area. As shown in FIG. 10(a), among the TOC, a 96 bit length record called a "Q channel" is prepared for each track and the respective Q channels are stored in accordance with the order of corresponding tracks in the program area (In the above figure, a Q channel "Q.sub.i " corresponds to a i-th track "T.sub.i " from the head).
Each Q channel has a data structure as illustrated in FIG. 10(b). The value stored in each field (namely, Control, ADR . . . ) of a Q channel is a value having a respective meaning when a track is sought or data is read out from the track. For example, A PSec field is used to indicate the type of a disk format. Table 1 shows the relationship between a PSec value and a format type. Specifically, if PSec=00 h, the disk has a CD-DA (herein, including CD-Graphics), CD-ROM track, or a hybrid type of the CD-DA and CD-ROM tracks: if PSec=10 h, the disk has a CD-l format type: if PSec=20 h, the disk has a CD-ROM XA format type. In general, the tracks having different PSec values are not provided on the same disk. See FIG. 11.
Further, the Control field is used for indicating the types of corresponding tracks. FIG. 12 shows the relation between the values of Control and the types of tracks. As understood from the FIG. 12, if a second significant bit of the Control field is 0, the corresponding track is a CD-DA track or CD-Graphics track: if the above mentioned bit is 1, the corresponding track is a CD-ROM track. In other words, by referring to the PSec or the Control of the Q channel, whether the corresponding track is a CD-DA track or not can be identified.
A CD-ROM drive of the type which is internally installed in or externally optionally connected to a personal computer is connected to a CPU via a bus, in a similar manner to other peripheral devices, and receives commands issued from application or an operating system (OS) during a transaction via the bus. The main contents of commands toward the CD-ROM drive are for reading out CD-ROM data, for reproducing; tune-selecting; reproduction-stopping CD-DA data or the like. The exchange of the commands or data between the CD-ROM drive and the computer main body side are usually performed under an I/O transfer method. There are exemplified, as the standards of interface for connecting the CD-ROM to the personal computer, an Enhanced IDE (Integrated Drive Electronics) and SCSI (Small Computer System Interface), etc. There is also a CD-ROM drive of the type which, when audio data is to be reproduced, does not transfer audio data to the computer main body but reproduce audio sound read audio data by using an internal audio data reproducing circuit and a speaker.
CD-DA data is read out from a CD in accordance with the same manner as that of a conventional reproduction of a record. Briefly, the CD-DA data is generally read out at a "standard speed" (about 150 KBps) equal to a sampling frequency in order to reproduce it as a normal sound (that is, the data is reproduced without buffering data). On the other hand, CD-ROM data can be read out at a higher speed under buffering data or the like. A CD-ROM drive of the type, has also made an appearance, which can read out the CD-ROM data at four times speed (4.times.150=600 KBps), and eight times speed (=8.times.150=1.2 MBps).
The CD-DA format type is specified in the "Red Book", the CD-ROM format type is specified in the "Yellow Book" and the CD-I format type is in the "Green Book", respectively. The CD-I format is a standard for making an interactive operation possible and the CD-ROM XA format is a standard by which audio data is integrated with motion picture data and the obtained data is processed as multimedia data.
Recently, the needs of a PC user have been varied. For example, one of them resides in that he or she wants to make an access to CD-ROM data with keeping the performance of music CD (namely, continuously processing CD-DA data) in the case that a hybrid CD type is loaded. Especially, the PC main body side can easily perform the processing of reading out the CD-ROM data during the reproduction of the CD-DA data under the control of an operating system (for example, "OS/2" of IBM Corp. ("OS/2" is a trademark of IBM Corp.) or "Windows 95" of Microsoft Corporation) which supports a multitask environment. For example, the PC main body side can supply a "CD-ROM data reading out command" according to another program, while the processing of a previously supplied "audio reproducing command" is not terminated (that is, while the CD-ROM drive is carrying out an audio reproduction process). In other words, the PC main body side can request a "simultaneous reproduction" of audio data and computer data. If an enormously large program is to be installed into the PC main body, the needs for the "simultaneous reproduction" will be particularly large.
As described above, respective data of different data formats are stored in individual tracks (or different sessions) even on a hybrid type CD in which the CD-ROM data (namely, computer data) and the CD-DA data (namely, audio data) are mixed. Therefore, when a CD-ROM data reading out request is sent from the PC side during the reproduction of the audio data, a pick-up head must be moved to a CD-ROM track/session designated by a CD-ROM data read request from a CD-DA track/session which is being accessed at present, in order to execute the CD-ROM data reading out request. As a result of such head seek operation, the CD-ROM drive will have to interrupt the reading out of the CD-DA data and the reproduction of the audio data.
Most of conventional CD-ROM drives are designed to perform single task, so that they have to abort a processing of a prior command in order to perform the next command. Accordingly, when a conventional CD-ROM drive receives a CD-ROM data reading out request during the reproduction or play processing of audio data, the CD-ROM drive inevitably stops a CD-DA data reading out and reproducing operation. Further, since the conventional CD-ROM drive is not designed to automatically resume a command processing which is temporarily subjected to the abort processing, an audio data reproduction processing is not resumed even after the CD-ROM data is completely read out.
The PC main body side under a multitask environment can supply a next command during processing of a previous command. However, the CD-ROM drive only processes commands one by one. In other words, while the "simultaneous reproduction" can be realized on the display screen (desktop) of PC side, the CD-ROM drive cannot perform the "simultaneous reproduction".
Although the reading speed of the computer data has been promoted to increase step by step in such a way as to start from two times speed to, four times speed, eight times speed . . . , the audio data must have been read out at a standard speed equal to a sampling, because the audio data (CD-DA data) has needed to be reproduced as a normal sound at a standard speed. Specifically stated, the high speed type CD-ROM drive has needed to switch a data reading speed between a case where the computer data is read out and a case where the audio data is read out. This problem has been also an factor owing to which the simultaneous reproduction of the computer data/audio data has been prevented from being realized.